The Obama administration will not prosecute US intelligence officials involved in harsh interrogations of terror suspects, the president pledged on Thursday.

The assurance came as the Justice Department released four secret memos used during the Bush presidency offering legal justification for interrogation techniques that human rights experts classify as torture, such as waterboarding.

The action comes after weeks of heated debate within the administration over whether to release the memos. Some officials were concerned that public disclosure might help Al Qaeda and build momentum for investigation of alleged acts of torture by US intelligence officials.

Many human rights activists have urged the president to authorize an investigation of torture allegations during President Bush's war on terror, with some calling for the appointment of an independent prosecutor.

But the president and Attorney General Eric Holder have decided against that. "This is a time for reflection, not retribution," President Obama said in a statement released by the White House. "We have been through a dark and painful chapter in our history. But at a time of great challenges and disturbing disunity, nothing will be gained by spending our time and energy laying blame for the past."

Human rights activists blasted the decision.

Within days of taking office, Mr. Obama ended the use of harsh interrogation techniques described in the memos.

One memo, dated August 2002, authorized 10 special interrogation techniques for use against Al Qaeda suspect Abu Zubaydah, including waterboarding. Under this interrogation technique, the suspect is placed on a board or table with his feet above his head, a cloth is draped over the nose and mouth, and water is poured over his face.

The technique, widely considered a form of torture by human-rights experts, triggers an intense, uncontrollable sensation of drowning.

In the memo, then Assistant Attorney General Jay Bybee acknowledged that waterboarding came close to violating the US torture statute because it constitutes "a threat of imminent death." But he added that it would not amount to torture unless the experience resulted in "prolonged mental harm" lasting months or years.

"In the absence of prolonged mental harm, no severe mental pain or suffering would have been inflicted, and the use of these procedures would not constitute torture within the meaning of the statute," Mr. Bybee wrote.

Questions had been raised about Mr. Abu Zubaydah's mental health, based in part on his actions in court proceedings at Guantanamo.

ACLU lawyers said they were pleased the documents were released, but criticized the president's decision not to prosecute. "Enforcing the nation's laws should not be a political decision," said ACLU executive director Anthony Romero. "These memos provide yet more incontrovertible evidence that Bush administration officials at the highest level of government authorized and gave legal blessing to acts of torture that violate domestic and international law," he says.

In addition to waterboarding, the 2002 Bybee memo authorized slapping, pushing, confinement in a small, dark space, painful stress positions, and sleep deprivation for up to 11 days. It also approved a request to lock Abu Zubaydah in a confinement box with an insect.

The memo says: "You have informed us that he appears to have a fear of insects. In particular, you would like to tell Abu Zubaydah that you intend to place a stinging insect into the box with him."

Bybee says the plan would be to trick Abu Zubaydah into thinking he was about to be stung. "You have orally informed us that you would in fact place a harmless insect, such as a caterpillar, in the box with him," Bybee wrote.

"Through these memos, Justice Department lawyers authorized interrogators to use the most barbaric interrogation methods, including methods that the US once prosecuted as war crimes," Jameel Jaffer, director of the ACLU's National Security Project, said in a statement."

The president and attorney general emphasized that US officials who acted reasonably and relied on the legal advice in the memos would not be charged and face a criminal trial for torture.

Attorney General Holder said he'd informed the CIA that the government would provide legal representation, at no cost to government officials, in any legal proceeding concerning illegal interrogation methods. The pledge extends to international and foreign tribunals, Holder said.

The government would also indemnify any employee for any money damages ordered paid to torture or abuse victims and would provide representation in any congressional investigation.

"It would be unfair to prosecute dedicated men and women working to protect America for conduct that was sanctioned in advance by the Justice Department," Holder said.

Amnesty International executive director Larry Cox criticized the administration's decision not to prosecute. "The Department of Justice appears to be offering a get-out-of-jail-free card to individuals who, by US Attorney General Eric Holder's own estimation, were involved in acts of torture," Mr. Cox said in a statement.